1. Field of the Invention
The present invention relates to a silicone resin composition that is useful as an encapsulating material for optical semiconductor elements, and an optical semiconductor device that uses the composition.
2. Description of the Prior Art
In recent years, high-brightness LEDs that exhibit strong light intensity and generate significant heat have become commercially available, and these high-brightness LEDs are now starting to be widely used, not only as the BLU devices within liquid crystal display televisions and mobile phones, but also within general lighting applications. JP 2010-1335 A (Patent Document 1) discloses that by adding an epoxy-containing silicone to a phenyl-containing silicone resin, the adhesiveness can be improved, enabling an encapsulating material to be provided that exhibits excellent heat resistance, light-resistant stability and weather resistance. Further JP 2009-527622 A (Patent Document 2) discloses a silicone resin composition comprising a phenyl-containing resin and a linear phenyl-containing hydrogenoligoorganosiloxane which is useful for extending the life of an LED.
In an LED package, a structure known as a lead frame that is prepared by silver plating a copper sheet is used. Depending on the conditions under which the LED is used, a sulfur fraction may be produced at the lead frame. It is known that if the gas permeability of the silicone used as the encapsulating material is high, then silver sulfide is produced at the silver surface over time, causing blackening of the surface. It is also known that this blackening of the lead frame causes a reduction in the brightness of the LED. Accordingly, low gas permeability is a desirable property for the encapsulating resin. Although phenyl-containing silicones are said to exhibit particularly low gas permeability even among silicone resins, it was discovered that when the compositions disclosed in the above Patent Documents 1 and 2, each of which uses a phenyl-containing silicone resin as the base component, were subjected to a sulfide test, the resistance to discoloration was unsatisfactory. It is generally accepted that the resistance to gas permeability can be improved by increasing the cross-linking density, but if the hardness of the cured product becomes too high, then other problems such as wire breakage or chip detachment can occur.
Patent Documents 3 to 15 also disclose phenyl-containing silicone compositions that are suitable for use with LEDs.    Patent Document 1: JP 2010-1335 A    Patent Document 2: JP 2009-527622 A    Patent Document 3: JP 2000-198930 A    Patent Document 4: JP 2003-128922 A    Patent Document 5: JP 2005-42099 A    Patent Document 6: JP 2005-105217 A    Patent Document 7: JP 2006-63092 A    Patent Document 8: JP 2006-335845 A    Patent Document 9: JP 2007-8996 A    Patent Document 10: JP 2007-39483 A    Patent Document 11: JP 2007-191504 A    Patent Document 12: JP 2008-1828 A    Patent Document 13: JP 2008-19385 A    Patent Document 14: JP 2008-50494 A    Patent Document 15: JP 2009-242627 A